The water in my tank is evaperating, I think I've lost about 1 gallon. I was wondering, if I could take out another gallon then add 2 gallons and that would work as a cycling or whatever it's called. lol

I also have a live plant, well the bulbs, and I think it's molding. how does it do that?

Hmmm... I won't pretend to be a beta fish expert, but I do know how difficult it is to keep live plants in a fish tank. I've had to replace several over the nine months I've had my fish. Plants need lots of light and bulbs are hard to start, so it probably did die and started rotting. If you can afford it, you can replace your bulbs with a plant that's already sprouted or do what I did in one of my three tanks, and replace the live plant with a fake one. Betas love plants that allow them places to hide and "Prowl" around. At least mine do.
As for the evaporation, how fast does the water evaporate? If it's going very fast, there might be a leak in your tank system.

the bulbs never even grew roots. I have one plant that did grow, actually I had 2 that did grow until someone uprooted one of my plants. I'm not sure how fast it evaporates, I've had it filled for about 2 1/2 weeks now, it's been really hot in my house. No air conditioner yet.

Cycling is a bacterial growth that changes toxic ammonia to toxic nitrite to non-toxic nitrate. Search up cycling, or if you do not have a fish in the tank yet, fishless cycling onto your local web browser

It doesn't hurt anything. I do it a lot. Sometimes I do partial changes on mine when I don't really have time to do a full change.Just dip out some old water and replace it with clean water and you don't even have to take the fish out. I have small tanks though.